The Lost Wax Process of Casting Bronze Explained

Above, left then clockwise: In 2002, we sculptor instructors at the Elizabet Ney Sculpture Conservatory in Austin, Texas, Marla Ripperda, Bill Barnett, and Kelly Borsheim pour molten bronze from the crucible. Nice chaps, eh? Next: Ten, large bronze wall-hanging sculpture; Detail of Warrior Spirit; and Shield, large wall hanging sculpture [also available small, the size of a door pull].

The Lost Wax Bronze Casting Process - Sculpture

Casting bronze is not at all like carving stone. We all understand how stone sculpture is created: the artist cuts away rock. He has one chance at it. Whatever is left is what you see as the art. Not so with a bronze sculpture. While each artist and each foundry (the place where bronze casting is done) works in a different way, most use the “Lost Wax Method” of bronze casting. It takes artists years to fully understand the process and challenges of casting molten bronze. I will try to sum it up in simple terms here.

L-R: Sculptor Kelly Borsheim sculpts at a show as demonstration in Dallas [May 1999]; Enjoying the Texas bluebonnets while working at home in the spring [April 2002]; In the studio working on Together and Alone [2001]

Second, a mold is made of the original art. This may be a one-part mold or a two-part mold, depending on the complexity of the art. A two-part mold [the norm] consists of an inner mold of soft, flexible rubber (usually silicone) that is capable of retaining find detail and an outer mold that is rigid.

The rubber mold wraps around the unusual shapes of the art and is flexible enough to be peeled away from the art or future castings. The outer mold is made of a hard, non-flexible material (such as plaster or fiberglass, I prefer the latter) that supports the inner mold to prevent any distortion when hot, liquid wax is later poured into this mold. This supporting function is why this outer mold is nicknamed "the mother mold."

Each work of art creates different considerations in the design of the mold. The moldmaker must understand the entire process in order to make a quality mold that will withstand many castings.

Using gravity, the first [print] coat of rubber is applied. It is a thinner layer than the ones to follow so that the rubber flows into the surface details and may render with as much accuracy as possible the sculptor's vision. In this case, the rubber is tinted red. What color is not important. It is only important that you see the difference so that you do not miss any section.

The next layer is thicker. It must be applied early enough so that it attaches well to the rubber underneath it. And then the sculpture is turned as needed to apply to every section until the project is completed. The piece you see here [above] is called I am You [2004], and you may see more of the mold-making process on that page below images of the finished bronze.

Here you see John Borsheim applying the second layer of silicone rubber on the Shield [2004]. I know it is the second layer because he always added a colorant to every other rubber layer so we would know for sure that there was equal coverage on all parts of the sculpture.

I must admit, there are not many people like John when it comes to moldmaking. He is a mechanical engineer and one of the smartest people I know. I mean, for example, look at this image and the one above it. He concocted these setups to be able to hold and rotate the clay sculpture in order to make the molds while not damaging or shifting the forms at all! He is incredibly thorough and I daresay his molds might be called a work of art.

Above: The flexible inner mold of Eric [2004], is finished. You see here the shims, made of cut-up waxed paper cups, that will separate the sections for the outer non-flexible [mother] mold. Note the "keys" we have put in the base. These will help the inner mold stay connected to the mother mold when the mold is upside down during the pouring of the wax later.

Note: the rubber mold is made in such a way that its outer form becomes a simple shape with no undercuts. That allows the future mother [non-flexible] mold to pull away from the flexible inside so that the cast wax may be safely removed. Careful planning happens to make sure that each part of the mother mold pulls away in one direction without problem.

Above: Again with I am You [2004], we proceed to the mother mold. As with the rubber mold, each layer of the fiberglass mold is colored to help us keep track of our progress. [Actually... one layer is left the natural golden color, the other layer we added some red]. To the left you see the first fiberglass layers being applied to the sculpture.

Image on the right, above: After some hardening, the fiberglass is removed from the inner mold, reassembled [so as not to lose the form... insurance, although the fiberglass should be hard enough already]. The inside may continue to air out. Also note that the fibers that stick out during the application process were ground off once hard and dry. No point having any painful sharp edges around!

Fiberglass is messy, stinky, and dangerous for your body to breathe in. John and I worked together always on the fiberglass mold. We worked in the car port, under cover from rain, dropping leaves, or even hot sun, but open to the air. We wore masks and gloves, and naturally old clothes.

I cut the dry fiberglass cloth from the roll and then used an old heavy papercutter to cut pieces into small squares and rectangles. I prefer to have these pieces cut ahead of time because once the resin is mixed up, we must move fast. I tried to keep my rubber-gloved hands dry while my job was to feed the fiber mats into the bowl of resin, timing it as I watched John's gloved hands take the wet fiber squares and press them into the sculpture, smoothing out any air bubbles to make sure that the fiberglass forms a close shape around the rubber mold. It was a big help that between our four hands, two remained dry.

I prefer fiberglass to plaster molds [often reinforced with metal] despite its toxicity because fiberglass is not only lighter, but it is stronger. This mold will be flipped in all sorts of directions and tapped onto the countertop to insure that the hot wax enters every part and evenly so, without air bubbles. Plaster tends to chip and when it does, it may enter the mold or otherwise contaminate the process. In that sense, fiberglass is cleaner [once cured]. But I will say that I prefer to keep the fiberglass outside until most of the stink has evaporated.

Above: Some Borsheim wax sculptures waiting to be sprued and then cast into bronze using The Lost Wax Method.

Each foundry works slightly differently, as each brain does. I usually consult with the foundry of choice for any given project to find out how to make the best mold for that foundry's operation. When casting Against the Dying of the Light [2005], we decided that the small arms needed to be cast solid. The body and head would be hollow, but the piece would stop in the hip area. Then the fourth piece would consist of the lower part of the man's body, as well as the large hands pulling him down would be all one piece, and hollow. To the left, you may see these waxes that were cast from the molds. They await the next step in the process.

It is important that these waxes have a consistent thickness [within each piece] because bronze will replace the wax. As bronze cools, it shrinks. Thick pools of bronze "lava" will retain heat longer than thinner sections of molten bronze. Shrinkage may cause surface marks or even unfilled areas, as the cooling metal will pull material from neighboring parts. It is a serious concern for a quality sculpture and will be minimized with a consistent thickness of the wax.

Above: Here are images of Against the Dying of the Light [2005], in the cast wax (left) and then after the bronze has been cast and cooled and removed from the fired investment mold. The metal bar that you see inside the bottom of the sculpture was welded in after the fact, but it could have been added as a wax bar to the wax cast. This is to allow for mounting the bronze sculpture to, in this case, a stone base.

The sprue is basically a wax funnel with feed lines and vent lines. The sole purpose of the sprue is to create a pathway for the molten bronze to flow through in order to create the intended bronze sculpture. Here I am seen welding my wax sculptures Seer [2011], and the figures in The Unwritten Future [2011] to the sprue.

A lot of thought goes into how the sprue is designed, as one considers gravity and how liquid fills a hollow space. One must avoid creating air pockets where a liquid cannot flow or the bronze sculpture will be missing parts. Each sprue is different for each art work or combination of small artworks on a sprue.

This photo to the left was the year of my divorce [2011] and thus, I decided to take the local bronze casting class to understand the setup in this new school [instead of my usual foundry]. It was very minimalist, but functional for my small works.

Although it is called The Lost Wax Method of casting metal, the fact is that you may use anything that burns out of the mold cleanly and disappears. So, here instead of using thin strands of wax as air vents [for air to escape as the bronze chases it through the space that was once wax, I will explain more in the next images], the teacher had us use wax-coated strings. This is enough to allow air to get out of the way of the bronze, but being thin, also saves on how much bronze is used. The additional benefit to that is much less cleanup later, when chasing / cutting off the extra bronze.

In this commercial foundry in Italy, you may see each of the stages of the dipped sprues into the second mold, often called an investment mold or the ceramic shell mold. The colors range from white to pale yellows to deeper yellows. The color will immediately tell the foundrymen how many times a sprue has been dipped into the ceramic shell slurry. After enough layers have been made and the slurry has dried, this investment mold will be fired, and the wax will melt out of the mold. Now, you understand why this complicated process is called "the lost wax method" to casting metal.

After the wax sprue with sculptures has been dipped in enough coats of the ceramic shell slurry (thick enough to support the force and temperature of molten bronze [about 2000 degrees Fahrenheit]), the mold is ready to be fired. During this process, the wax [and any other materials in the sprue] is melted out of the mold, leaving an air space where once was wax. It is this part of the process that inspired the name "The Lost Wax Process/Method" in casting metal.

This image left shows the ceramic mold after firing. The sculpture is now just an air space. If the mold is dropped before the pour, the sculpture is lost. This second mold is a one-shot deal: after the bronze is cast and cools to solid metal again, this mold is then broken away to reveal the bronze. This is one reason that the first mold is made. Can you imagine having done all of this work to then lose it all because of an accident?

Ingots of bronze await melting in the crucible.

Above left: In the center of the image is an insulated cover that is hoisted with pulleys. It is an oven and is used to heat the ceramic mold so that its temperature is closer to the temperature of the liquid bronze that will soon be poured into it. We do not want to shock this mold into cracking or the bronze will flow all over the sand pit that supports the mold. This can create lovely abstract patterns of bronze that some of my friends have used in their art making, but it totally negates all of the work done for the intended sculpture!

The middle and right images show the pouring of molten bronze. The container is called a crucible and the color is a gorgeous orange! Note that the mold is placed into a tub of sand. Sand will move around the shape of any mold to support it upright for the pouring. Also, in the event that the ceramic mold develops a leak, well-gloved hands can scoop up some sand and throw it on the mold to stop/cool the bronze from leaking out.

The size of the sculpture or I should clarify, the sprue, is determined by the size of the crucible. If you were to try to melt more bronze and pour it on top of the already cooled bronze, there would be a freeze line through the entire piece. Instead large sculptures are cast in manageable pieces and later welded together.

The mold is set aside in a safe place [i.e., not burning whatever it rests upon] and the bronze eventually cools to a solid form. Once this happens, the ceramic shell mold is broken away to reveal the treasure within. Here you see one of my bronze turtles emerging in the mixed stone and bronze Sea Turtles I [1997].

Left is another sprue after casting. Everything that was wax is now bronze. Using metal-working tools, the sculpture must be cut out of this bronze sprue and re-carved or cleaned up (chased) at each connection point of sculpture and feed or vent lines. If the sculpture was large and cast in parts, these parts will be welded together before chasing.

Above, a frog is being welding onto the bronze cattails, since each part of the garden sculpture Rock Towers and Frogs [2016] was cast separatly.

The artisan Noris paints on chemicals to give the bronze sculpture a lovely patina. He heats the metal with the torch in his other hand. This only happens after all welding and handling have been done. Patination is the last stage and, many foundrymen feel, the most creative step in their work at the foundry.

Here you may see several small bronze artworks in the process of receiving a patina, including The Unwritten Future.

Once all of the metalwork is finished, the bronze sculpture is thoroughly cleaned, usually by blasting with tiny glass beads (sand is too coarse and would damage fine detail). This removes any last traces of ceramic shell mold, as well as the oils from human hands. A patina (or colored chemical finish) is then applied, usually with the aid of a torch. In addition, all of my bronzes have added layers of wax, or more often, a product called Incralac. They protect the patina of the new sculpture, allowing one to touch and enjoy.

Note the torch table in the background left.

Each bronze sculpture created by the lost wax casting process is considered to be an original work of art because each one takes so much work unique to the piece.

Above, a torch is used to create a patina on the bronze Warrior Spirit [2016],

Above, 9-1-1 [2001], left to right: the original wax sculpture modeled on September 12, 2001; then a plaster copy; and then the bronze sculpture with a patina to remember the ashes that made every New Yorker appear the same.

Above, my bronze wall-piece [bas relief] Ten [2002], came about eight years after I made a 1-minute gesture sketch from a model. I never got it out of my head, such is the power of a great pose and model!

Each bronze sculpture created by the lost wax casting process is considered to be an original work of art because each one takes so much work unique to the piece.

Above and below, the enlarged version of a popular bronze, Ten [large] [2003], was cast as one piece!

Below you may see that we added [welded] a couple of options of how to secure the bronze to a wall or a gate.

Although I enjoy sculpting in clay (terra-cotta), I liked the idea of metal sculpture and
needed to try my hand at it. The bronze medium allows an artist to sculpt thin, extending parts
(such as arms and fingers). I like the grace and movement I can try to put in my bronze work.

There is a whole art unto itself in casting and finishing bronze. I was fascinated by the molten bronze as we poured it out of the crucible and into our investment molds. I learned the basics of welding and felt the thrill of using power tools as I shaped the cooled metal. And it is exciting to see how chemicals change and color the metal as the patina is applied--and even as the sculpture ages.

Many of the pages of the individual sculptures will include some text and images of the work as it was being created. If you would like to see more of my bronze sculpture work, please click here. Thank you for visiting.